Traditionally, as a sensor unit equipped with an inertial sensor which detects inertia based on a predetermined detection axis, a sensor unit 91 (device) having a configuration in which an angular velocity sensor 83 is installed inside a box-like case 80, as shown in FIG. 27, is known. More specifically, a configuration in which a substrate 82 with the angular velocity sensor 83 mounted thereon is directly fixed to a bottom surface 81 inside the box-like case 80 is employed. The angular velocity sensor 83 has a comb-drive actuator 84 having a spindle or comb-shaped electrode, formed on a semiconductor substrate using a MEMS (micro electro-mechanical systems) technique, and electrically reads out a motion generated by a Coriolis force that acts when an angular velocity is applied (as a capacitance change, for example).
Meanwhile, such a traditional configuration has a problem that it is susceptible to the influenced of characteristic vibrations (noise vibrations) transmitted from an installation target surface 85 (device) and that their influence on detection accuracy cannot be denied. For example, in the case where the sensor unit 91 is installed in a car navigation system, there is a risk that noise vibrations due to engine operations of the automobile may be directly transmitted from the bottom surface 81 of the case 80 to the angular velocity sensor 83. This is a problem not limited to the package configurations of angular velocity sensors but common to the package configurations of inertial sensors as a whole, such as acceleration sensors.
In view of the foregoing problem, JP-A-2006-194681 proposes the sensor devices shown in FIG. 28 to FIG. 30. A sensor device 92 of FIG. 28 has a configuration in which the case 80 is in an upside-down (vertically inverted) state and in which the substrate 82 including the angular velocity sensor 83 is suspended with metal springs 86 from the bottom surface 81 of the case 80. Meanwhile, a sensor device 93 of FIG. 29 has a configuration in which the substrate 82 including the angular velocity sensor 83 is suspended from an outer peripheral edge of the case 80, using flexible substrates 87. Also, a sensor device 94 of FIG. 30 has a configuration in which a stair-like step 88 is formed at a peripheral edge part inside the case 80 and in which a plurality of bonding wires 89 extends from the step 88 and suspends the substrate 82 including the angular velocity sensor 83. According to this literature, the influence of noise vibrations can be sufficiently attenuated with the configurations of the sensor devices 92 to 94.
However, the sensor devices 92 to 94 of JP-A-2006-194681 have a problem that it is difficult to achieve stable detection accuracy because of low reliability. More specifically, in any of the configurations of the sensor devices 92 to 94, the position of the substrate 82 including the angular velocity sensor 83 is in the state of floating inside the case 80 and therefore the angular velocity sensor 83 tilts by its own weight or due to change with time. As the angular velocity sensor 83 tilts, a bias is applied to the comb-drive actuator 84 because of the influence of gravity, and therefore its influence extends to the detection result, causing low reliability.
Also, in the case of the configuration in which the substrate 82 is suspended with the springs 86, there is a risk that the substrate 82 may vertically vibrate due to the influence of noise vibrations. This vertical vibration, too, may influence the detection result, making it difficult to achieve stable detection accuracy. The sensor devices 93, 94, too, employ the configurations in which the substrate 82 is suspended with elastic members. Therefore, the vibration of the substrate 82 occurs due to the influence of noise vibrations and it is similarly difficult to achieve stable detection accuracy.